1. Field of the Invention
The present invention generally relates to an exposure apparatus and an exposure method. In particular, the present invention relates to an exposure apparatus and an exposure method for performing projection and exposure on an object to be exposed such as a single crystal substrate for a semiconductor wafer or a glass substrate for a liquid crystal display (LCD).
2. Related Background Art
Up to now, in manufacturing a device (e.g., a semiconductor device, a liquid crystal display device, or a thin film magnetic head) using a photolithographic technique, a projection exposure apparatus has been adopted. The projection exposure apparatus projects and transfers a circuit pattern drawn on a mask or a reticle (in this application, the two terms are used interchangeably) onto a wafer etc., by using a projection optical system.
As regards the projection exposure apparatus, there is an increasing demand for projection and exposure of the circuit pattern on the reticle to the wafer with a higher resolving power as may keep up with recent miniaturization and high integration scale of an integrated circuit. The smallest possible size (resolution) of the pattern which the projection exposure apparatus transfers is proportional to a wavelength of light used for the exposure but inversely proportional to numerical aperture (NA) of the projection optical system. Accordingly, the shorter the wavelength, the higher the resolution. Thus, in recent years, as the light source, an ultrahigh-pressure mercury lamp (g-line (wavelength: about 436 nm) or i-line (wavelength: about 365 nm)) is replaced by a KrF excimer laser (wavelength: about 248 nm) or an ArF excimer laser (wavelength: about 193 nm) which has a shorter wavelength. Further, an F2 laser (wavelength: about 157 nm) is being put into practical use. In addition, a demand to further enlarge an exposure region is growing.
To meet such demands, a step-and-scan system exposure apparatus (also called a “scanner”) is gaining popularity over a step-and-repeat system exposure apparatus (also called a “stepper”). The stepper collectively exposes a substantially square exposure region on a wafer after the reduction, whereas the scanner relatively scans the reticle and the wafer at a high speed with the exposure region formed in a rectangular slit-shape to thereby expose a large-area screen with accuracy.
The scanner effects correction such as alignment of a wafer surface with an optimum exposure position upon exposing a predetermined position of the wafer by measuring a surface position of the wafer at the predetermined position by surface position detection means of an oblique optical system before the predetermined position of the wafer comes in an exposure slit region during the exposure. Thus, it is possible to suppress an influence of a levelness of the wafer.
As shown in FIG. 16, in particular, plural measurement points (K1 to K3) are arranged on each of a preceding region 510 and a succeeding region 520 of an exposure slit region 500 in a longitudinal direction (i.e., a direction orthogonal to a scanning direction) of the exposure slit with an intention to measure a tilt as well as a height (focus) of the surface position of the wafer. Here, exposure scanning light is moved from both the preceding region and the succeeding region. Therefore, the measurement points are arranged in the preceding region and the succeeding region of the exposure slit region so that the focus and the tilt of the wafer can be measured prior to the exposure. Various methods of measuring the focus and the tilt have been proposed (see Japanese Patent Application Laid-Open No. H09-45609 (counterpart: U.S. Pat. No. 5,750,294 B), for example). FIG. 16 is a schematic diagram showing an example of arrangement of the measurement points K1 to K3 relative to the exposure region 500 in a conventional case.
Further, proposed as a method of measuring and correcting a surface position of a wafer in a scanner is a method of arranging plural measurement points in a previously scanning region outside the exposure region and measuring a focus and a tilt in a scanning direction and a non-scanning direction (see Japanese Patent Application Laid-Open No. H06-260391 (counterpart: U.S. Pat. No. 5,448,332 B), for example). Also proposed is a method of arranging plural measurement points in the exposure region, obtaining measurement information on a focus and a tilt in a scanning direction and a non-scanning direction, and correcting by moving the wafer (see Japanese Patent Application Laid-Open No. H06-283403 (counterpart: U.S. Pat. No. 5,448,332 B), for example).
In recent years, a wavelength of the exposure light has been more and more shortened and NA of the projection optical system has further increased, leading to an extremely smaller focal depth. A much higher precision, i.e., focus precision is being needed for aligning the wafer surface to be exposed with a best imaging plane.
In particular, there are growing needs for the precise measurement on the tilt of the wafer surface in the scanning direction (transverse direction of the exposure region) and the accurate correction of the tilt. The need for the enhancement of a property of following the focus in the exposure area of the wafer that has too rough (uneven) surface is also growing.
However, even if the surface position of the wafer is measured in the exposure region and corrected by moving the wafer, there is a defect in that on account of being subjected to scanning exposure, the wafer is corrected and moved too late for alignment of the wafer surface to be exposed with the best imaging plane.
Also, a method of arranging plural measurement points in a scanning direction and a non-scanning direction in the exposure region and obtaining information on tilt of the wafer in the scanning direction based on chronological information obtained through scanning of the wafer encounters a problem that measurements includes an asynchronous error to lower measurement precision, for example, making it impossible to align the wafer surface to be exposed with the best imaging plane.